Re: deadlkres() panic

2010/7/1 Bryan Venteicher <bryanv_at_daemoninthecloset.org>:
> On a recent -current, I got the following panic from deadlkres:
>
> Assertion wchan != NULL failed at /usr/src-nfs/sys/kern/subr_sleepqueue.c:680
>
> Tracing pid 0 tid 100058 td 0xffffff00024bf7a0
> kdb_enter() at kdb_enter+0x3d
> panic() at panic+0x176
> sleepq_type() at sleepq_type+0x56
> deadlkres() at deadlkres+0x224
> fork_exit() at fork_exit+0x12a
> fork_trampoline() at fork_trampoline+0xe
> --- trap 0, rip = 0, rsp = 0xffffff8074976d30, rbp = 0 ---
> (Hand transcribed, doadump() hung)
>
> deadlkres() came across a TD_IS_SLEEPING()'ing thread that was not a
> sleepqueue (ie, td->td_wchan == NULL).
>
> I don't think this is an invalid state for thread to be in: After adding itself
> to a sleepq and setting a timeout, the thread calls sleepq_timedwait_sig().
> sleepq_catch_signals() determines there is a signal pending so it removes the
> thread from the sleepq via sleepq_resume_thread(). Returning to
> sleepq_timedwait_sig(), in the call to sleepq_check_timeout(), the thread is
> unable to cancel the timeout because it is already firing (likely waiting on
> thread_lock()). So the thread calls TD_SET_SLEEPING() followed by mi_switch().
> deadlkres() then picks up thread_lock(), finding td is TD_IS_SLEEPING() &&
> !TD_ON_SLEEPQ().
>
> The attached patch takes care of the panic for me.

I think that your analysis and patch are both fine and are committed,
along with a small cleanup, as r209761.

Thanks,
Attilio


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